| 摘要 |
To provide an amplifier in which an amount of variation in output impedance between high- and low-gain modes is small and gain control can be accomplished accurately. A radio frequency signal inputted through an input terminal (1) is provided to a base of a transistor (102) and outputted to its collector. In a high-gain mode, a switch (110) is turned off to place a transistor (104) in an off sate. All radio frequency signals are inputted into an emitter of a transistor (103), outputted to its collector and then to an output terminal (2). In a low-gain mode, the switch (110) is turned on to place the transistor (104) in an on state so that a base voltage of the same value as that of the transistor (103) is applied to it. A current ratio between the transistor (103) and the base-grounded transistor (104) is determined by a size ratio between them. Therefore, a current can be accurately adjusted by adjusting the size ratio between them and outputted to the output terminal (2). Because load conditions of output in the high-gain mode and the low-gain mode are the same, the amount of variation in the output impedance is small.
|
